Retroreflective sheeting that reflects light in substantially the same direction as the incident direction has been used for signs such as road traffic signs and construction signs, vehicle registration plates, safety materials such as helmets and life preservers, markings such as those on signboards, and others. Hexagonal cube-corner retroreflective sheeting that is excellent in light reflective efficiency is known as one example of the retroreflective sheeting.
In producing the hexagonal cube-corner retroreflective sheeting, respective hexagonal cube-corner retroreflective elements are formed by die molding. Specifically, hexagonal cube-corner retroreflective sheeting having a large area is formed by using a die assembly including a plurality of dies in each of which the shapes of hexagonal cube-corner retroreflective elements are arranged in a closest-packed state.
Patent Documents 1 to 4 listed below teach cube-corner retroreflective sheeting produced by using a die assembly including a plurality of dies.
In the hexagonal cube-corner retroreflective sheeting taught in Patent Documents 1 and 2, boundaries between dies in the assembly are provided along edge lines of hexagonal cube-corner retroreflective elements. Accordingly, boundaries between regions corresponding to respective dies in produced hexagonal cube-corner retroreflective sheeting are also along the edge lines of hexagonal cube-corner retroreflective elements. Furthermore, the hexagonal cube-corner retroreflective elements are oriented in the same direction in each die as those in the other dies in the assembly. Accordingly, the hexagonal cube-corner retroreflective elements are oriented in the same direction over the entire produced hexagonal cube-corner retroreflective sheeting.
In the method for manufacturing hexagonal cube-corner retroreflective sheeting taught in Patent Document 3, a die assembly includes a plurality of dies having a triangular or quadrangular shape and the die assembly is used to form hexagonal cube-corner retroreflective sheeting. There is no disclosure as the specifically what direction each of the dies for hexagonal cube-corner retroreflective elements is oriented in the die assembly.
In addition, in the method of manufacturing hexagonal cube-corner retroreflective sheeting taught in Patent Document 4, a die assembly includes a plurality of dies in which each boundary therebetween is provided along an edge line of a hexagonal cube-corner retroreflective element and the die assembly is used to form hexagonal cube-corner retroreflective sheeting, or a die assembly including a plurality of dies having a triangular or quadrangular shape and the die assembly is used to form hexagonal cube-corner retroreflective sheeting.
It is known that hexagonal cube-corner retroreflective sheeting has rotation angle characteristics that turning the retroreflective sheeting on a plane relative to a light incident direction in a state in which light is incident on the surface of the retroreflective sheeting in an oblique direction changes the light reflective efficiency according to the turning angle of the light incident direction with respect to the retroreflective sheeting.
In order to improve such rotation angle characteristics, hexagonal cube-corner retroreflective sheeting including two or more types of hexagonal cube-corner retroreflective elements having a relationship in which the retroreflective elements rotate with each other around an axis perpendicular to the surface of the retroreflective sheeting has been proposed.
Hexagonal cube-corner retroreflective sheeting taught in Patent Documents 5 to 7 has a plurality of regions in which hexagonal cube-corner retroreflective elements formed in regions adjacent to each other are oriented in directions rotated by 180 degrees with respect to each other. According to the hexagonal cube-corner retroreflective sheeting taught in Patent Documents 5 to 7, since the rotation angle characteristics of mutually adjacent regions differ by 180 degrees, the reflection characteristics for light incident in directions different from one another by 180 degrees are improved in the retroreflective sheeting as a whole. Thus, the rotation angle characteristics of 180 degrees are improved in the retroreflective sheeting as a whole.
[Patent Document 1] U.S. Pat. No. 3,417,959
[Patent Document 2] U.S. Pat. No. 3,922,065
[Patent Document 3] U.S. Pat. No. 3,924,929
[Patent Document 4] Japanese Examined Patent Publication No. 46-25177
[Patent Document 5] U.S. Pat. No. 3,458,245
[Patent Document 6] U.S. Pat. No. 3,541,606
[Patent Document 7] Japanese Unexamined Utility Model Publication No. 63-109233